Radio communication devices require the generation of stable operating frequencies in order to function properly. Typically, stability has been obtained by using a crystal oscillator to provide a reference frequency. The reference frequency is used as a baseline for the frequency synthesizers in the radio. Specifically, the local oscillators of the radio are phase locked to the reference frequency. However, crystal oscillators by themselves can not provide a sufficiently constant frequency to meet the frequency stability requirements of the radio. In particular, the output frequency of a crystal oscillator varies over temperature.
Radios may also require frequency compensation to precisely center a radio transceiver operating frequency onto a base station channel frequency. This is accomplished using an automatic frequency control (AFC) which determines an error between the radio transceiver operating frequency and the base station channel frequency and applies a correction signal to the crystal oscillator to align the radio to the base station. However, the resolution needed to provide AFC correction (typically 50 Hz at 1 GHz) requires the use of a high resolution frequency adjust (warp) circuit in the crystal oscillator which adds cost to the radio.
Methods to temperature compensate crystal oscillators are known in the art. Generally, these methods incorporate compensation circuits composed of analog or digital devices and are used to provide a relatively constant output frequency over temperature. Typically, these circuits are incorporated within the crystal oscillator, and the output of the oscillator is applied as a reference frequency to all the frequency synthesizers or local oscillators of the radio.
A more recent temperature compensation scheme uses an uncompensated crystal oscillator and applies a temperature compensation signal directly to a divider of the phase locked loop of the frequency synthesizer, while at the same time the signal also incorporates the desired frequency multiplication for the frequency synthesizer. For this scheme, since the crystal oscillator is not temperature compensated, the output of the crystal oscillator can not be used to directly provide a stable reference frequency for other local oscillators of the radio unless each frequency synthesizer is temperature compensated by other means, a more costly approach. Furthermore, each frequency synthesizer would need to have a higher resolution than the resolution required to simply step between radio channels in order to provide adequate temperature compensation resolution, which adds cost to the radio.
There is a need for a radio communication device that replaces a temperature compensated crystal oscillator with temperature compensated local oscillators. There is also a need for a radio communication device that can be compensated at the local oscillators. The device should still meet requirements for high performance, low power, and low cost. The radio architecture should minimize the number of high resolution devices, realize both temperature and frequency compensation of local oscillators, and meet radio performance requirements.